1. Field of the Invention
The present invention relates to an imaging apparatus such as a CMOS image sensor and a processing method thereof.
2. Description of the Related Art
Imaging apparatuses such as electronic cameras that employ a memory card having a solid-state memory device as a recording medium are already known as imaging apparatus that image, record, and play back still images or moving images. These electronic cameras record and play back still images or moving images that are imaged with a solid-state imaging device such as a CCD or a CMOS.
When imaging using an imaging device, processing is carried out using dark pixel signals that are read out after performing charge accumulation in a similar manner to actual imaging in a state in which the imaging device is not exposed and actual imaging pixel signals that are read out after performing charge accumulation in a state in which the imaging device is exposed. It is thereby possible to perform dark noise correction processing. As a result, a high quality image can be obtained by correcting imaged pixel signals with respect to image degradation such as pixel defects caused by unique minute flaws of the imaging device, fixed pattern noise caused by a readout circuit, or dark current noise generated in the imaging device.
In a correction method that uses dark pixel signals, since a large-capacity memory for one frame is necessary for storing dark pixel signals, the cost of the imaging apparatus is increased. Also, since it is necessary to previously acquire dark pixel signals for one frame prior to actual imaging, a shutter release time lag increases and hinders optimum imaging. Further, since random components that are referred to as random noise are included in dark pixel signals, fixed pattern noise can not be completely suppressed by only performing processing to subtract dark pixel signals from actual imaging pixel signals.
To solve this problem, a method has been proposed that divides a pixel region into an effective pixel region and an ineffective pixel region, averages the pixel signals of the ineffective pixel region, and performs processing to subtract the obtained average value from the pixel signals of the effective pixel region. The effective pixel region outputs signals that accumulate charges generated according to incident light on the pixel region of the imaging device. The ineffective pixel region is a pixel region in which, for example, a pixel surface of the same circuit configuration as the effective pixel region is covered with a light-shielding film such as aluminum, and which outputs signals that are not dependent on incident light. According to this method, averaging parameters are increased by subjecting pixel signals of the ineffective pixel region to averaging processing, and random noise included in the correction signal is decreased.
An object of Japanese Patent Application Laid-Open No. 2002-016841 is to provide a solid-state imaging apparatus which generates a correction signal based on pixel signals of an ineffective pixel region having a plurality of rows to increase the reliability of the correction signal and suppress fixed pattern noise. To achieve the above object, a solid-state imaging apparatus is proposed in which a correction signal for one row portion is generated from light shielded pixels of a plurality of row portions and pixel signals of effective pixels are corrected.
Japanese Patent Application Laid-Open No. 2004-015712 proposes a solid-state imaging apparatus in which pixel signals obtained from pixel signals of an ineffective pixel region are totaled across a plurality of frames, and an average value is determined from that total value to generate a correction signal and suppress fixed pattern noise.
Japanese Patent Application Laid-Open No. 2000-138864 proposes an imaging apparatus in which reading of each photoelectric conversion element is performed a plurality of times within a single-field period, and charges of elements for detecting a vertical black reference are read per every reading that is conducted a plurality of times within a single-field period.
However, there are the following problems with the above described methods of suppressing fixed pattern noise according to the prior art.
According to the solid-state imaging apparatus proposed in Japanese Patent Application Laid-Open No. 2002-016841, there is the problem that when an ineffective pixel region with a large number of rows is provided to enhance the reliability of a correction signal, the chip area increases. More specifically, the influence of random noise included in a correction signal is reduced by providing a large number of rows in the ineffective pixel region used for averaging.
According to the solid-state imaging apparatus proposed in Japanese Patent Application Laid-Open No. 2004-15712, there is the problem that since pixel signals of an ineffective pixel region extending across a plurality of frames are required to generate a correction signal, the invention can not be used in an imaging mode that captures only a single frame such as when a single image is captured using a digital still camera. More specifically, the influence of random noise included in a correction signal is reduced by increasing the number of frames of pixel signals of an ineffective pixel region used for averaging.
According to the solid-state imaging apparatus proposed in Japanese Patent Application Laid-Open No. 2000-138864, the influence of random noise included in a correction signal is reduced by increasing the number of frames of pixel signals of an ineffective pixel region used for averaging. More specifically, there is the problem that since pixel signals of an ineffective pixel region extending across a plurality of frames are required to generate a correction signal, the invention can not be used in an imaging mode that captures only a single frame such as when a single image is captured using a digital still camera. Also, even in a case in which pixel signals of an ineffective pixel region that extends over a plurality of frames can be acquired as in a case of imaging a moving image, when the ineffective pixel region is small there is the problem that the response until the start of imaging worsens since a large number of frames are required. Further, there is the problem that when the number of frames to be integrated is decreased by increasing the ineffective pixel region, the chip area is increased.
To solve the above problems, an object of the present invention is to provide an imaging apparatus that reduces the influence of random noise included in a correction signal based on pixel signals of an ineffective pixel region of only one frame without increasing a chip area, as well as a processing method thereof.